It is generally sought to measure liquid levels in situations where one seeks to determine liquid volumes or changes in liquid volumes. The customary means of measuring liquid levels comprise electrodes which are partially immersed in the liquid. Suitable measurement systems are employed to measure the resistance and/or electrical conductivity of the liquid, which measurement is proportional to (inter alia) the liquid level (or liquid volume).
In this connection, the physical relationship LF=δ0·ZK·V is utilized, where LF is the electrical conductivity of the liquid, δ0 is the specific conductivity of the liquid, V is the volume of the liquid, and ZK is the so-called “cell constant”, which can be calculated from the ratio of the electrode distance and the wetted electrode surface area.
The specific conductivity δ0 influences the slope of the measurement curve (measured value as a function of liquid level); thus, prior to measurement of the liquid level one must perform a calibration measurement to determine this specific conductivity. Such calibration requires additional expense of resources, which one would like to avoid or minimize.
DE 197 26 044 A1 discloses a liquid level indicator, particularly for plant containers, which is comprised of an elongated rod with at least two mutually insulated electrodes and an electronic circuit connected to the electrodes via connecting lines disposed in said rod, wherewith the indicator is powered by a D.C. voltage source. The liquid level is indicated on a display device. This liquid level indicator system requires a plurality of electrodes which are disposed at different altitudes (elevations). Each electrode is connected to the electronic circuit by a separate connecting line. This system is quite costly to realize.
DE 40 42 257 discloses a method and device for determining the liquid levels of electrically conducting liquids. This is achieved via, e.g., a chain (series) of resistances which extend vertically into the liquid. From a measurement of the overall resistance of the system one can determine how many of the resistances are disposed above the liquid level, and what the height (“depth”) of the liquid level is; this is facilitated by the fact that the metal surfaces are applied along the surface in a close and constant mutual separation. This system too is costly, and delivers only discrete values of the liquid level.
DE 30 18 718 discloses an electrode device for determining liquid levels, which device also employs a plurality of liquid-level-determining electrodes, which are disposed on a support and are continuously displaceable in the longitudinal direction of the support.
JP 08050047 A discloses an electrode device having a plurality of electrodes, for measuring the electrical resistance of the liquid in order to determine the liquid level.
JP 2004077439 A discloses a measuring electrode for determining the liquid level, which electrode has a conical or hemispherical shape. The lower region of the electrode has a small diameter, and the upper end has a large diameter.
Liquid level measurements are also made in water filter devices, to aid in determining when the filter medium in a filter cartridge has become exhausted (is due for replacement). Such an exhaustion indicating device is disclosed in, e.g., EP 1 484 097 A1. This device is based on measurements of resistance of the liquid using two electrodes which are disposed one over the other in a water container or are disposed in a feed channel. A drawback of this device is that each time the water quality changes one must make additional calibration measurements.
Another exhaustion indicating device is disclosed in WO 01/74719; this device employs a plurality of electrodes which extend into the liquid. In or on the water filter device, an evaluation unit is disposed which determines the measured fill volume over a specified time interval, which evaluation unit is connected to a display unit, the so-called “exhaustion indicating device”. In such water filter devices, raw water is introduced into the feed funnel, and then passes downward through the filter cartridge, and leaves the cartridge on the underside thereof, and the filtered water is collected in a lower chamber of the device. With raw water being fed from time to time, and being drawn down, the liquid level varies numerous times during the service life of the cartridge; and the history of the liquid level enables one to determine the amount of liquid which has passed through the filter cartridge. Thus supplied with data on the amount of liquid which has been passed, the exhaustion indicating device can then present information as to the degree of exhaustion of the filter medium.